The prior art is replete with many disclosures of cementing in wells penetrating subterranean formations. In such operations, drilling fluids (often referred to as “muds”) are normally present in oil and other similar wells at the time of cementing a casing into a borehole with an aqueous cement slurry. Since the mud and cement are not always compatible with each other, it is often desirable to separate or prevent contact between them by use of a cement spacer system. Without the use of such spacer systems, the incompatibility of the aqueous cement slurry with the oil based mud is often of such severity that a mixture of the two forms an unpumpable mass. Such unpumpable masses may prevent displacement of at least a portion of the drilling fluid with a cement spacer system. Spacer systems also serve to water-wet the wellbore surfaces to promote bonding of the cement sheath to the wellbore and casing. Such cement spacer systems are often thought of as flushing agents.
In a typical operation, the cement spacer system is prepared at the site of entry into the well bore such that the density of the system matches or exceeds the densities of the fluids to be removed from the well and the like. Conventional mixing equipment for such applications usually takes the form of a 50 to 100 barrel “batch mixer”. A typical batch mixer is a hydraulically powered, self-contained unit which mixes the cement spacer formulation immediately prior to the cementing operation. Such units are equipped with large paddles to facilitate the mixing of the cement spacer, centrifugal pumps for circulation of the spacer within the batch mixer and pressurized bulk storage tanks for the transport of the weighting agent for the cement spacer. Batch mixers may be truck-mounted or skid mounted depending on wellsite location requirements.
Once at the drilling site, the spacer system must be prepared from its constituent materials (typically water, gelling agent and weighting agent) and combined with any additional materials (such as surfactants or mutual solvents). The cement spacer system must then be tested for the appropriate density, with the density adjusted to fall within the appropriate range for the particular well bore. Independent of the particular chemistry of the cement spacer system, the storage time of the finished spacer prior to pumping into the wellbore is generally limited to no more than 12 hours due to gravitational settling of the weighting agent.
It would be desirable to develop a cement spacer system that was storable and which remained as a pumpable fluid over extended periods of time. Such storable compositions would offer several advantages over the conventional systems of the prior art. Most notably, they could be made in advance and stored until needed. In addition, the storable compositions may be made at a different location from the job site. Such storable compositions may then be transported to the job site before the introduction of the cementitious slurry into the well bore. Further, the composition may be stored in inexpensive nonpressurized tanks and easily transferred. Still further, the mixing equipment used on the job site would only be required to homogenize the composition as required en route to the downhole pumps. Such storable compositions may further undergo quality control prior to introduction into the well bore.